Free space optical communication has attracted considerable attention recently for a variety of applications. Because of the complexity associated with phase or frequency modulation, current free-space optical communication systems typically use intensity modulation with direct detection (IM/DD). Atmospheric turbulence can degrade the performance of free-space optical links, particularly over ranges of the order of 1 km or longer. Lack of homogeneity in the temperature and pressure of the atmosphere, for example, can lead to variations of the refractive index along the optical transmission path. Such refractive index variations can deteriorate the quality of the received image and can cause fluctuations in both the intensity and the phase of the received optical signal. These fluctuations, which are also referred to as fading, can lead to an increase in the link error probability, limiting the performance of optical communication systems. Aerosol scattering effects caused by rain, snow and fog can also degrade the performance of free-space optical communication systems.
For optical communications systems design, the effects of fading must be accounted for to ensure reliable system operation. In suitable atmospheric conditions, received optical signals may be strong which could saturate a photo detector at the optical receiver. In poor atmospheric conditions, weak optical signals need to be boosted over background noise levels in order to properly distinguish transmitted data from noise. Thus, optical systems needs to be designed to operate over a fairly large dynamic power range accounting for strong signal strength under optimal conditions and weak signal strength under poor conditions. Previous attempts at addressing the dynamic range issues due to fading have focused on variable gain optical amplification systems which attempt to determine the atmospheric condition at the time and adjust the optical gain of the system accordingly. These systems however introduce path delays in the received optical signal which can lower overall system bandwidth. Such systems also require complicated control loops to adjust the gain which can add significantly to system complexity and cost.